The present invention relates generally to planar illumination devices and liquid crystal display devices using the planar illumination devices. In particular, the invention relates to an illumination device that uses a plurality of point light sources comprising solid light-emitting elements such as light-emitting diodes as illumination light sources for a liquid crystal display panel, mixes light from the point light sources and planarly emits the resulting white light or the desired resulting color light, and to a liquid crystal display device using the illumination device as a backlight.
A liquid crystal display device is equipped with a illumination device in order to visualize electronic latent images formed on its liquid crystal display panel except liquid crystal display devices using outside light. The illumination device is installed on the back or front of the liquid crystal display panel. In general, the illumination device installed on the back of the liquid crystal display panel is called a backlight and the illumination device installed on the front of the panel is called a frontlight.
Some backlights for small-sized handheld terminals such as cellular phones include a light guide having a cold-cathode fluorescent lamp disposed on the side surface (side edge or incident surface) thereof as with lap-top computers with a relatively large display screen. However, backlights suited to handheld terminals use a single of or a plurality of point light sources, solid light-emitting elements, typified by light-emitting diodes with small power consumption in place of cold-cathode fluorescent lamps. Incidentally, the backlights using such solid light-emitting elements are applied to a display monitor for a lap-top computer or other handheld terminals and to a display of a television receiver. A description is here made using a light-emitting diode (hereunder, also abbreviated as an LED) as a solid light-emitting element by way of example.
The known backlights include a side edge type in which a light source is installed on the side edge of a light guide plate and a directly-beneath type in which a light source is installed directly on the back of a liquid crystal display panel. A backlight using LEDs is characterized by good color reproducibility and generally employs the side edge type using a light guide plate in view of reducing a thickness, weight and power consumption. The side edge type backlight using LEDs is configured as a surface light source as below. The LEDs are attached on the side surface of a transparent plate (light guide plate) made of acryl plate or the like. In addition, while being propagated in the light guide plate, the light emitted from the LED changes its course to the surface of the light guide plate. The backlight of this type is disclosed by e.g. Japanese Patent Laid-open Nos. 2004-200072 and 2004-6187.
In addition, the LEDs disclosed in the patent documents mentioned above are attached to the side surface of the light guide plate so that those light-emitting axes may be parallel to the plain surface of the light guide plate. Japanese Patent Laid-open No. 2004-133391 discloses that a special lens is attached to the light-emitting surface of an LED so that light is emitted in a direction perpendicular to the light-emitting axis thereof.